Heavy-Metal Contamination, Transfer Factors, and Health-Risk Assessment in Roadside Soils and Crops Along a Major Highway in South Kazakhstan

The Shymkent–Saryagash–Abay (A-15) international highway is a major Kazakhstan–Uzbekistan freight corridor that runs through the irrigated horticultural belt of the Turkestan Region in South Kazakhstan, where adjacent fields supply vegetables and cucurbits to the regional market. Composite soil samples (n = 18) were taken at six distances (2–300 m) from the road edge across three locations during 2022–2023, along with edible fruits of tomato, cucumber, watermelon, and melon (n = 12) from the adjoining fields. Pb, Zn, and Cd were measured via flame atomic absorption spectrometry after HNO3/H2O2 digestion. Soil concentrations decreased sharply with distance (Pb: 26.3 → 5.98 mg kg−1; Zn: 21.29 → 4.16; Cd: 0.47 → 0.01 mg kg−1), exceeding the national soil MPCs by 1.5–3 times within 2–10 m. Pb and Zn exceeded the Kazakhstani food-safety MPCs in all four crops, and Cd in three of four (tomato, cucumber, and melon). Transfer factors followed the order of Cd (2.90–4.40) > Zn (1.99–3.00) > Pb (0.16–0.30), and the Cd geo-accumulation index ranged from 1.05 to 1.65 at 2–5 m. Adult dietary risk was acceptable (HI = 0.029–0.052; CR < 1.7 × 10−6), yet food-safety exceedances support a precautionary sanitary buffer and combined soil-and-crop monitoring along the corridor.